EP0221467B1

EP0221467B1 - Vibrating type transducer

Info

Publication number: EP0221467B1
Application number: EP86114770A
Authority: EP
Inventors: Michihiko Fuji Elect. Ltd. Tsuruoka; Wataru Fuji Elect. Ltd. Nakagawa; Noriomi Fuji Elect. Ltd. Miyoshi; Naohiro Fuji Elect. Ltd. Konosu; Tadao Fuji Elect. Ltd. Hashimoto
Original assignee: Fuji Electric Co Ltd
Current assignee: Fuji Electric Co Ltd
Priority date: 1985-10-25
Filing date: 1986-10-24
Publication date: 1990-08-01
Also published as: DE3673121D1; US4872335A; EP0221467A1

Description

The present invention relates to a device for measuring the density or the pressure of a fluid on the basis of the resonant frequency of a vibrator as set forth in the preamble of Claim 1. A device of this kind is known from EP-A-0 015 368.
In this document an air pressure transducer is disclosed having a chamber in which a circular diaphragm is disposed which is in contact with shallow air cushions on both sides and is surrounded by stiff walls. This air pressure transducer utilizes the fact that the stiffness of the air cushions on both sides of the diaphragm influences the resonant frequency of the diaphragm. In order to attain a high measuring accuracy and in order to prevent calibration instabilities, the diaphragm is formed from a stiff material and is secured at the peripheries thereof between rigid walls by means of spacer rings having a thickness that the return force created by the air cushions in the space within the spacer rings at atmospheric prsssure is substantially three to ten times the return force of the diaphragm. The diaphragm is excited by an induction coil. Any other teaching concerning the dimensions of the chambers separated by the diaphragm are not disclosed in the reference.
Another conventional vibrating type transducer for measuring the density of a fluid on the basis of the resonant frequency of a vibrator is shown in Figure 1 as it is disclosed in US-A-3,677,067. In Figure 1, a measuring tube 1 is arranged within a fluid channel in which a fluid being monitored flows in bhe direction of G. The measuring tube 1 is normally arranged that its axial direction conforms to a direction of flow. A vibrator, 4 in the form of a rectangular plate having edges on both sides fixed to the inner surface of the measuring tube 1 to include its central axis is fixed to one end of a cylindrical body 5 whose central axis is perpendicular to the surface of the vibrator 4. A cylindrical connecting case 6 is fixed to the other end of the body 5. The cylindrical body 5 is provided with a threaded portion 3 for fitting the measuring tube 1 into the fluid channel in the manner above described. The vibrator 4 is made to promote self-excited flexural vibration resulting from arcuate bending on the edges not fixed to the measuring tube 1 by means of a mechanism (not shown), so that the vibrator 4 vibrates at the resonant frequency (Fn) of the vibrating system including the vibrator 4. When the measuring tube 1 is arranged in the fluid channel, the fluid being monitored is passed through the measuring tube 1 and the fluid in contact with the vibrator 4 is also caused to vibrate as the vibrator 4 experiences flexural vibration. The mass of the vibrating system including the vibrator 4 increases to the extent of the mass of the fluid being monitored when the fluid vibrates because of the vibrator 4. As a consequence, the value of the above resonant frequency (Fn) is different from that when the fluid is not in contact with the vibrator 4. The resonant frequency (Fn) is expressed by Eq. (1).
where M=mass of vibrator 4; K=spring constant; and MI=mass of fluid as the above additional mass.
K, M in Eq. (1) are constants independent of the properties of the fluid, whereas MI corresponds to the density of the fluid. As is apparent from Eq. (1), the density of the fluid can be measured by monitoring the frequency of vibration (Fn). The device shown in Figure 1 is arranged so that the density can thus be measured and the relation of the density p of the fluid to Fn conceptually becomes what is shown in Figure 2.
In Figure 2, Fno represents a value of Fn when p=0, i.e., when the vibrator 4 is within a vacuum. When the fluid being monitored is a gas, the mass MI of the gas vibrating as the vibrator 4 vibrates is significantly smaller than that of the vibrator 4. Because a gas has compressive properties and a low density p the slope of the plot of Fn versus p is very small. It is consequently difficult to measure the density of gas by means of the density measuring instrument shown in Figure 1 because the frequency change corresponding to the density change is insufficient. Another problem is that the temperature range applicable to the density measuring instrument is narrow because the frequency (Fn) fluctuates as a result of changes in the dimensions of the vibrator 4. When the ambient temperature changes become greater than the change of the frequency (Fn) corresponds to the change of the density of the gas this effect cannot be disregarded, provided that the ambient temperature changes sharply.
The present invention is intended to solve the above problems inherent in such conventional devices, specifically it is therefore an object of the invention to provide a vibrating type transducer capable of accurately checking a low density fluid such as gas having compressive properties over a wide range of measuring temperatures.
The above-noted objects are achieved by the characterizing features of Claim 1. Preferred embodiments of the invention are subject matter of the dependent claims. It is also preferred that the first fluid inlet comprise a conduit having a cylindrical bore in flow communication with the first chamber. It is also preferred that the device include a resilient member affixed to the diaphragm with means for adjusting the resilience of that resilient member.

Brief description of the drawings

Figure 1 is a perspective view of a conventional vibrating type transducer.
Figure 2 is an operational diagram of the transducer of Figure 1.
Figure 3 is a vertical sectional view of a first embodiment of the present invention.
Figure 4 is a block diagram of a detection circuit for driving the first embodiment.
Figure 5 is a drawing explanatory of the operation of the first embodiment of the present invention:
Figure 5(A) is a typical diagram of the vibrating system; and Figures 5(B), (C) are electric equivalent circuits of the vibrating system, respectively.
Figure 6 is a plot showing density-frequency test results obtained from the first embodiment of the present invention.
Figure 7 is a vertical sectional view of a second embodiment of the present invention.
Figure 8 is a vertical sectional view of a third embodiment of the present invention.
Figure 9 is a diagram explanatory of the operation of the third embodiment of the present invention:
Figure 9(A) is a typical diagram of the vibrating system; and
Figure 9(B) is the electric equivalent circuit of the vibrating system.
Figure 10 is a plot showing density-frequency characteristic test results obtained from the third embodiment.
Figure 11 is a plot showing frequency-impedance test results obtained from the third embodiment.
Figure 12 is a vertical sectional view of a fourth embodiment of the present invention.
Figure 13 is a vertical sectional view of a fifth embodiment of the present invention.
Figure 14 is a vertical sectional view of a sixth embodiment of the present invention.
Figure 15 is a vertical sectional view of a seventh embodiment of the present invention.
Figure 16 is a sectional view taken on line X-X of Figure 15.
Figure 17 is a vertical sectional view of an eighth embodiment of the present invention.
Figure 18 is a schematic diagram showing the principal part of the eighth embodiment.
Figure 19 is a plot showing temperature-rate of frequency change test results obtained from the eighth embodiment.

Description of the preferred embodiments

Referring now to the accompanying drawings, an embodiment of the present invention will be described in detail. In the drawings, like reference numbers or letters designate parts performing similar functions.
Figure 3 is a vertical cross-sectional view of an embodiment of the present invention, and Figure 4 is a block diagram of a detection circuit used in the above embodiment. A closed-end cylindrical vibrating member 8 shown in Figures 3, 4 is provided with a piezo-electric vibrator 9 adhesively bonded to its inner bottom surface 8a. The vibrating body further includes a collar portion 8c and a flange portion 8b at its outer peripheral edge. The vibrating means 8 is formed from a metal sheet about 0.1 [mm] thick of kovar or 42 Ni-Fe alloy having a low thermal expansion coefficient. As shown schematically in Figure 4 the piezo-electric vibrator 9 consists of a discoidal piezo-electric wafer 9a, 0.1 to 0.2 mm thick. A first electrode 9b is formed on one side of the wafer 9a, a second electrode 9c and a third electrode 9d are formed on the other side thereof. That side on which the first electrode 9b is installed is affixed to the bottom surface 8a of the vibrating member 8 and connects the electrode 9b to the vibrating member 8 electrically. The vibrating member 8 and the piezo-electric vibrator 9 comprise the diaphragm 10. A container 11 is provided with internal threads 11a on the inner face on the open end side and one end of a cylindrical projection 12 is fixed to the surface 11 b of the container 11 such that the cylindrical body 12 is coaxial with the container 11. A circular opening 11c through the projection 23 communicates with the inside of the container 11.
A closed-end cylindrical housing 13 is provided with external threads 13a on the outer side and, by screwing the external threads 13a into the internal threads 11a, the flange portions 8b of the vibrating member 8 is sandwiched in between the housing 13 and the container 11 and fixed in an inner cavity formed by the housing 13 and the container 11. A first chamber 14 is defined by the diaphragm 10 and the container 11, whereas a second chamber 15 is defined by the diaphragm 10 and the housing 13. Openings 16 are formed in the bottom 13b of the housing 13 and a printed circuit board 18 forming a detection circuit 17 is bonded to the inner surface of the bottom 13b. The first chamber 14 is separated by the diaphragm 10 from the second chamber 15 in a fluid tight condition.
Conductors 19a are led out of the chamber 15 through the opening 16 to connect the detection circuit 17 to those outside the second chamber 15. Lead wires 19b, 19c, 19d connect the vibrating member 8 and the electrodes 9c, 9d of the piezo-electric vibrator 9 to the detection circuit 17, respectively. The vibrating means, here the diaphragm 10, the container 11 and the cylindrical projection 12 constitute a sensor 20.
The construction and operation of the detection circuit 17 will subsequently be described. An amplifier 21 applies output voltage to the piezo-electric wafer 9a through the electrode 9c and a feedback circuit 22 detects the voltage generated in the piezo-electric wafer 9a through the electrode 9d and positively feeds it back to the amplifier 21. The vibrating diaphragm 10 shown in Figures 3, 4 is constructed such that the piezo-electric wafer 9a expands and contacts in the radial direction when a.c. voltage is applied across the electrodes 9b, 9c. As a result the expansion and contraction of the piezo-electric wafer 9a causes the bottom 8a of the vibrating member 8 to vibrate in the axial direction of the cylindrical housing 13. Consequently, a.c. voltage corresponding to the distortion of the piezo-electric wafer 9a is again generated across the electrodes 9d, 9b and the voltage is positively fed back to the amplifier 21 through the feedback circuit 22. Ultimately, the diaphragm 10 continues the vibrating state in which it resonates at a natural frequency F through self-excited vibration.
An impedance conversion circuit 23 receives the output a.c. voltage 21 a of the amplifier 21 having a frequency equal to the natural frequency F to facilitate the conversion of the voltage into a signal as described later. A waveform-shaping circuit 24 subjects the output signal of the converter circuit 23 to waveform shaping and outputs a signal 24a which is a pulse train of frequency F. The above-described amplifier 21, the feedback circuit 22, the impedance conversion circuit 23, the waveform-shaping circuit 24 and the printed circuit board 18 mounting these elements constitute the detection circuit 17.
A signal conversion circuit 25 receives the signal 24a and supplies a signal corresponding to the frequency F of the pulse train forming the signal to an operational means 26, which produces a density signal 26a by performing operations on the output signal of the signal conversion circuit based on an operational equation as described later.
When the sensor 20 shown in Figure 3 is arranged in the fluid being monitored, the fluid being monitored is introduced into the chamber 14 through the cylindrical projection 12 and the chamber 15 through the openings 16. The diaphragm 10 vibrates at the resonant frequency of what is termed the vibrating system 40 which consists of the first chamber 14 into which the fluid being monitored has been introduced, the opening 11c, the inside of the cylindrical projection 12 and the diaphragm 10 when the diaphragm 10 causes self-excited vibration. The frequency F of the pulse train of the output signal 24a produced by the waveform-shaping circuit 24 accordingly becomes equal to the resonant frequency of the vibrating system 40. The vibrator 9, the detection circuit 17 and the signal conversion circuit 25 constitute a frequency detector 27 shown in Figure 4 for detecting the resonant frequency F of the vibrating system 40.
Referring to Figure 5, the configuration of the vibrating diaphragm 10 will be described. As shown in Figure 3, the collar portion 8c of the vibrating member 8 is arranged opposite the inner side wall of the first container 11 with an extremely narrow gap therebetween. The volume of the chamber 15 is significantly greater than that of the chamber 14 to make the pressure inside chamber 15 almost nearly free from fluctuation even though the diaphragm 10 thus vibrates, so that the natural frequency of chamber 15 is significantly lower than that of the vibrating system 40 composed of the diaphragm 10, the chamber 14, the opening 11c and the cavity inside the cylindrical projection 12. As a consequence, the principal components shown in Fgiure 3 are represented by Figure 5(A) in a typical form. Figure 5(A) illustrates the mass Mm of the diaphragm 10, the area S of the bottom surface 8a of the vibrating member 8 and a mechanical compliance Cm proportional to the spring constant Km of the diaphragm 10, which establish a relationship given by Cm=_1/Km, where Ma=mas_s of the fluid being monitored within the cylindrical projection 12; and Ca is expressed by Eq. (2).
where W=volume of the chamber 14; X=acou_stic velocity in the fluid being monitored; and p=density of the fluid being monitored.
In Figure 5(A), the height h of the chamber 14 and the sectional area S₁ of the bore of the cylindrical projection 12 are made extremely small to the extent that the principal parts are so arranged to allow the mass of the fluid being monitored in the chamber 14 and the acoustic volume within the bore of the cylindrical projection 12 to be negligible. Accordingly, the vibrating system constructed as shown in Figure 5(A) is represented by an electrical equivalent circuit shown in Figure 5(B) as the result of the conversion of the acoustic vibrating system consisting of the chamber 14 and the cavity within the bore of the cylindrical projection 12 into a mechanical vibrating system.

where Mao and Cao=mass and acoustic compliance given by Eqs. (3), (4), respectively.
Assuming that Eq. (5) is established by setting the angular frequency of the vibration at w, Figure 5(B) is rewritten as Figure 5(C) and Eq. 6 is set up when the resonant frequency of the circuit of Figure 5(C) is F.
Eq. (7) is obtained from Eqs. (3) (6).
As is obvious from Eq. (6), the diaphragm 10 resonates at the frequency F if the principal components shown in Figure 3 are so arranged as to establish the equivalent circuit of Figure 5(C) and the frequency F of the pulse train forming the output signal 24a of the waveform-shaping circuit shown in Figure 4 has a value corresponding to the mass Ma of the fluid being monitored in the cylindrical projection 12. Acordingly, the density of the fluid being monitored can be obtained by determining the value of the frequency F. The operational circuit 26 performs operations based on Eq. (5) and outputs the density signal 26a equivalent to the density p of the fluid being monitored. As shown in Eq. (6), the mass Ma changes as the density of the fluid being monitored changes and the resulting mass Ma affects the frequency F in such a manner as to enlarge the effect by (5/5,)² times since the principal components are formed for satisfying S/S,»1. Thus (S/S,)^Z»1. When the principal components of the device are adapted to set up the circuit of Figure 5(C), the transducer shown in Figures 3, 5 is, as is apparent from the comparison between Eqs. (6), (1), capable of accurately monitoring fluid such as gas having a low density or compressive properties.
As shown in Figure 6, the principal components of the transducer shown in Figures 3, 4 is made up to set the value of Mao/Mm as large as possible to increase the slope of the frequency v. density curve and thus improve the measuring sensitivity. Accordingly, Eq. (8) is obtained from Eq. (7).
The above description is justifiable provided that Eq. (8) is valid. Eqs. (5)-(8) are therefore applicable because the principal components are adapted to increase the compliance Cm of the diaphragm 10 by forming the vibrating member 8 and the portion of the container adjacent to where the vibrating member 8 is sandwiched in between the container 11 and the housing 13, to make the acoustic compliance Cao of the chamber 14 significantly smaller than the compliance of the diaphragm 10 by minimizing the height h of the chamber 14, and maximizing Mao/Mm. In such a transducer, accordingly, the frequency of the pulse train represented by the equivalent circuit of Figure 5(C) and forming the signal 24a of Figure 4 becomes equal to F and, since S/S,»1, the transducer will monitor a low density fluid such as gas with high sensitivity and accuracy. The frequency of the diaphragm 10 changes as the temperature of the fluid being checked changes but the effect of such a change on the measuring accuracy is less than that in case of the transducer of Figure 1, so that the fluid can be monitored over a wide temperature range.
A characteristic line R shown in Figure 6 reflects the test result when the sensor 20 of Figure 3 is arranged to the satisfaction of all the above conditions. In Figure 6, the resonant frequency is equal to the natural frequency of the vibrating system 40 consisting of the diaphragm 10, the first chamber 14, the opening 11 c and the cavity within the cylindrical projection 12 or to the frequency of the pulse train forming the output signal 24a of the waveform-shaping circuit shown in Figure 4. Given a 0.05 [mm] thick plate as the vibrating member 8, a 24 [mm] outer diameter cylinder, a 0.5 [mm] high (h) chamber 14, a 3 [mm] inner diameter of the bore of the cylindrical projection 12 and that 15 [mm] long (I(shown in Figure 5(A)) cylindrical projection 12, the characteristic line R holds. In this case, the compliance Cm of the diaphragm 10 is set at 0.51x10-⁴ [m/N] and the acoustic compliance Cao of the chamber 14 at 6.3x10-⁶ [m/N], whereby the conditions of Eq. (7) are met.
A characteristic line S represents the test result when the sensor 20 is so arranged to set the dimensions of the diaphragm 10 at the same values as those defined above and dispense with the chamber 14 and the bore of the cylindrical projection 12, i.e., when a transducer is constructed in a manner similar to what has been applied to the conventional one shown in Figure 1. As shown in Figure 6, the characteristic line S is slightly inclined, whereas the characteristic line R is steeply inclined. It is thus clear from Figure 6 that the use of the sensor exhibiting the characteristic line R makes it easier to measure a low density as compared with the use of what has the characteristic line S. A characteristic line T of Figure 6 shows the test result when a mechanical acoustic vibrating system such as the conventional known piezo-electric vibrator wherein a container for making a cavity communicate with one side of the mechanical acoustic vibrating system, the container being provided with a simple opening. In such a vibrating system, the cavity formed therein is relatively large in size, so that the acoustic compliance of the cavity is large in contrast to the small compliance of the mechanical diaphragm rigidly formed. Consequently, the characteristic line T is inclined slightly up to the right. The mechanical acoustic vibrating system is therefore unusable for measuring the density as shown in Figure 6.
Figure 7 is a vertical cross-sectional view of a portion corresponding to what is shown in Figure 3 in a second embodiment of the present invention. What is different from the embodiment of Figure 3 is that the embodiment of Figure 7 includes a chamber 28 and a cylindrical projection 29 respectively similar to the first chamber 14 and the cylindrical projection 12, however, the chamber 28 and the cylindrical projection 29 are provided on the side onto which the piezo-electric vibrator 9 is attached to the diaphragm 10. In this case, the bottom portion 30b of a housing 30 that defines the chamber 28 together with the diaphragm 10 is raised to minimize the thickness of the platelike chamber 28. Each electrode of the piezo-electric vibrator 5 is connected to the detection circuit 17 installed outside the chambers 28, 14 through a terminal 31 passed through the bottom 30b. One end of the cylindrical projection 29 is fixed to the bottom 30b to allow the interior thereof to communicate with an opening 30c bored in the bottom 30b. A sensor 32 consists of the components shown in Figure 7 excluding the detection circuit 17, the signal conversion circuit 25 and the operational means 26. Because the sensor 32 in Figure 7 is thus constructed, the diaphragm 10 arranged in the fluid being monitored and caused to vibrate permits the sensor 32 to sense nearly the sum of the mass of the fluid in the cylindrical projection 29, whereas the mass of the fluid being monitored which vibrates as the diaphragm 10 vibrates, is almost equal to the mass of the fluid being monitored in the cylindrical projection 12 and sensed by the sensor 20 shown in Figure 3, even though that mass is added to the xass of the diaphragm 10. As is apparent from Eq. (6) the use of the ssnsor 32 makes it possible to measure the density with sensitivity higher than that in the case of the sensor 20 of Figure 3.
Although a description has been given of the application of the present invention to density measurement, the present invention is also applicable to pressure measurement. Although the above description refers to providing the container 11 with the cylindrical projection 12, and the housing 30 with the cylindrical projection 29, these cylindrical projections 12, 29 can be dispensed with, whereby the action of the fluid in the cylindrical projections 12, 29 can be replaced with that of the fluid in the openings 11c, 30c without problems.
Ma in Eq. (5) is expressed by Eq. (9).
where I=Iength of the cylindrical projection 12; and p=density of the fluid being monitored. Accordingly, the density can be obtained from Eqs. 6, 9, provided that the frequency F is known.
The frequency F changes according to Eq. (6) in the transducer of Figure 3 and it is obvious from Eqs. (6), (9) that the change of p is (S².I)/S, times amplified to cause the change of F. In the transducer thus constructed, accordingly, the advantage is that highly sensitive measurement can be made possible by increasing (S' - 1)/S_l. In addition to that advantage, low density fluid such as gas can also be monitored by installing the cylindrical projection 12 offering greater (S² _. I)/S,.
However, the cylindrical projection 12 thus installed causes acoustic resistance therein because of the viscosity of the fluid being checked and this phenomenon results in the reduction of Q of the vibrating system shown in Figure 5(A), i.e., the vibrating system consisting of the diaphragm 10, the chamber 14 and the fluid being checked within the cylindrical projection 12. The reduced Q increases the acoustic resistance in the cylindrical projection 12 and makes it significant whenever an attempt is made to increase measuring sensitivity by increasing or decreasing S, of the cylindrical projection. In consequence, it is attempted to increase measuring sensitivity in the transducer of Figure 3, Q will be reduced, thus preventing the diaphragm 10 from causing stable self-excited vibration. Consequently, stable density measurement becomes impossible.
Figure 8 is a vertical cross-sectional view of a third embodiment of the present invention wherein measures to counter the above problems posed in the first embodiment of Figure 3 have been taken. In Figure 8, there is installed a horn-shaped upper member 50 with one end attached to the container 11 instead of the straight-pipelike cylindrical projection 12. The bore of upper member 50 is shaped like an exponential horn whose cross-sectional area varies exponentially with the sectional position.
Because the transducer of Figure 8 is thus constructed, a signal 24a has a pulse train frequency equal to the resonant frequency of a vibrating system 52 compressed of the fluid being monitored in the chamber 14, the fluid contained in the opening in (the fluid in the member 50 and the chamber 14) hornlike upper member 50, the acoustic vibrating system 51 and the diaphragm 10 when the transducer is arranged in the fluid with its diaphragm 10 causing self-excited vibration. An electric equivalent circuit of the vibrating system 51 is shown in Figure 9(B). Figure 9(A) is a typical drawing of the vibrating system 52 corresponding to Figure 5(A) and, as shown in Figure 5, the acoustic compliance Cao of the chamber 14 obtained from Eq. 4 is so arranged as to be significantly smaller than the mechanical compliance Cm of the diaphragm 10. In Figure 9, the mass of the fluid being checked in the horn-like upper member 50 is expressed by Mb; a conversion factor for converting the acoustic impedance in the acoustic vibrating system 51 into that of the mechanical vibrating system by a; and acoustic resistance in the acoustic vibrating system 51 with the thus converted impedance of the mechanical vibrating system by R. The vibrating system 52 is represented by the equivalent circuit of Figure 9(B) in the vibrating type transducer of Figure 8 and apparently the resonant frequency F, of that circuit becomes what is defined by Eq. (10).
Therefore, the frequency of the pulse train signal 24a produced by the detection circuit 17 becomes equal to F, and, in the transducer of Figure 8, the density of the fluid being monitored can be measured by measuring the pulse frequency of the signal 24a. In the equivalent circuit of Figure 9(B), the density measurement may be unstable as Q of the vibrating system 52 will be reduced if the resistance R has a large value. Since R in Figure 9 is based on the acoustic resistance in the exponential horn-like upper member 50, however, the value R becomes substantially smaller than that in the case of the transducer of Figure 3 whose cylindrical body is shaped like a straight pipe, below the cutoff frequency in the cylindrical projection. Accordingly, Q of the vibrating system 52 shown in Figure 9 becomes larger than Q of the corresponding vibrating system shown in Figure 5(A) and the diaphragm 10 in the transducer of Figure 8 is free from instability. This results in stable density measurement.
Figure 10 displays test results obtained from the transducers of Figures 3, 8. Figure 10 shows the relationship between the density of the fluid being monitored and the resonant frequency F, shown in Eq. (10), wherein G=characteristic line of the transducer of Figure 8; H=c_haracteristic line of the transducer of Figure 3. Figure 10 attests the fact that, even if the shape of the upper projection varies, the inclination of the characteristic line, i.e., density mesuring sensitivity, can be equalized by properly increasing or decreasing the dimensions of the principal components.
Figure 11 shows other test results obtained from the transducers of Figures 3, 8. A characteristic line V in Figure 11 indicates the relationship between the impedance and frequency when the resonance circuit side is viewed from power suply terminals 53, 54, whereas a characteristic line W represents the relationship between the corresponding impedance and the density in the case of the transducer of Figure 3. The series and parallel resonance states still appear as the frequency changes in both the characteristic lines V, W because the static capacity across the driving electrodes in the piezo-electric vibrator 9 is connected to each resonant circuit in parallel in those tests. As is apparent from the drawings, however, the impedance changing mode in the characteristic line V is greater and steeper than that in the line W. According to the test results, Q of the vibrating system 52 in the transducer of Figure 8 is larger than Q of the vibrating system of the transducer of Figure 3. In the cases of Figures 10, 11, the adoption of the horn- like upper member 50 also makes available a vibrating type transducer allowing for stable measurement without reducing measuring sensitivity.
Figure 12 is a vertical sectional view of a fourth embodiment of the present invention. The difference from what is shown in Figure 8 is that an upper projection 55 having a conical horn-like opening instead of the exponential horn-like upper projection 50 is installed. Through tests, the present inventors have determined Q in the vibrating system employing such an upper projection 55 becomes slightly smaller than that in the case of the upper projection 50. The advantage is, however, that the transducer of Figure 12 is readily constructed because the shape of the opening in the upper projection 55 as compared with that of the upper projection 50.
In the third and fourth embodiments, the acoustic vibrating system combined with the diaphragm 10, e.g., the vibrating system 51, is attached to one side of the diaphragm 10 but the arrangement thereof is not limited to the above examples according to the present invention. The acoustic vibrating system may be installed on both sides of the diaphragm and needless to say diaphragms, each having a horn-like opening for introducing fluid, may be arranged on both sides thereof in this case.
In the transducer shown in Figure 3, both the mass Mn and compliance Cm of the diaphragm 10 vary with the shapes and dimensions of the vibrating member 8 and the piezo-electric vibrator 9 constituting the diaphragm. As is apparent from Eq. (6), each value of Mm, Cm therefore varies according to the transducer, thus causing a difference in performance among them.
Figure 13 is a vertical cross-sectional view of another embodiment of the present invention wherein measures to counter such variations have been taken into consideration. In this embodiment, a cylindrical upper projection 60 for introducing the fluid being monitored into the container 11 through the opening 11c c consists of an outer cylindrical body 61 having one end tightly fixed to the outer side of the container 11. The outer cylindrical body 61 includes internal threads formed on the inner surface and an inner cylindrical body 62 is screwed into the outer cylindrical body 61.
As is apparent from Eq. 6, the frequency F changes as the mass Ma of the fluid being monitored in the cylindrical upper projection 60 changes. When the diaphragm 10 does not resonate at the frequency corresponding to the given density of the fluid because there exists a difference in performance among transducers attributable to variations in Mm, Cm, it becomes possible to make the diaphragm 10 resonate at the given frequency by changing the mass Ma. In other words, the difference in performance among them can be nullified by changing the mass Ma, i.e., that difference in performance becomes readily eliminated in the case of the embodiment of Figure 13 by adjusting the length of the inner cylindrical body 62 screwed into the outer cylindrical body 61 so as to sequentially change the mass Ma. When such a transducer is applied to the measurement of the intake air density of an internal combustion engine, it is possible to obtain a transducer free from differences in performance without minimizing variations in the mass Mm and compliance Cm. The resonant frequency F of the diaphragm 10 can conform to a given value by adjusting the position of the inner cylindrical body 62 while the internal-combustion engine is operated in the normal air condition at 20°C and 1013 hPa.
Figure 14 is a vertical cross-sectional view of an additional embodiment of the present invention, which is similar in some respects to the embodiments of Figures 7 and 13. The difference between Figures 14 and 13 includes the configuration of the chamber 28 and a cylindrical lower projection 63 respectively corresponding to the chamber 28 and a cylindrical projection 29 of the embodiment of Figure 7. As in the case of the example shown in Figure 7, a closed-end cylindrical container 30 corresponding to the housing 13 of Figure 13 forms the chamber 28 together with the diaphragm 10. The housing 30 has a bottom 30b elevated close to the pizeo-electric vibrator 9 to reduce the acoustic compliance of the chamber 28 by shortening the dimension between the bottom 30b and the diaphragm 10. Each electrode of the vibrator 9 is connected to a detection circuit arranged outside the chambers 28, 14 through a terminal 31 passed through the bottom 30b. The lower cylindrical projection 63, as in the case of the upper cylindrical projection 60 of the embodiment of Figure 13, consists of an external cylindrical body 64 having one end fluid tightly fixed to the outer face of the bottom 30b of the container 30. The external cylindrical body 64 has internal threads and an inner cylindrical body 65 is screwed into the external cylindrical body 64. The lower cylindrical projection 63 is so arranged to introduce the fluid being monitored into the chamber 28 through a circular opening 30c provided in the bottom 30b, whereas the inner diameter of the inner cylindrical body 65 is equal to the diameter of the opening 30c. The difference in performance by types deriving from the above mass Mm and compliance Cm may be nullified using either cylindrical body 62 or 65.
Figure 15 is a vertical cross-sectional view of a seventh embodiment of the present invention and Figure 16 is a sectional view taken on line X-X of Figure 15. In Figures 15, 16, the flange 8b of the vibrating member 8 is molded into a plastic ring-like frame 73 in one body. As in the case of each of the above embodiments, the closed-end cylindrical container 11 is provided with the internal thread 11 a on the inner surface. The open end has a circular opening 11 within the cylindrical projection 12. The cylindrical projection 12 has one end fluid tightly fixed to the outer face of the bottom 11 b of the container in such a manner as to make the cylindrical projection 12 and the opening 11c concentric, the inner diameter of the former being equal to that of the latter. A circular step 11 1d is provided on the inner wall of the container 11 and a plurality of projections 11 are projected from the side wall having the stepped portion 11d.
A retainer 113 has external threads 113a its outer surface and a large diameter opening 113c in the bottom thereof. The retainer 113 supports a ring-like spring 72 having an alternate wave shape in the circumferential direction. The diaphragm 10 includes a frame 73 on its peripheral edge fixed to the container 11 as the frame 73 is pressed against the stepped portion 11 by spring 72 when the retainer 113 is screwed into the container 11 having the internal thread 11a. In other words, the vibrating member 8 is fixed to the housing 11 at the stepped portion 11d through the resilient support mechanism 71 consisting of the collar portion 8c of the vibrating means, the flange portion 8b and the frame 73 of the vibrating member 8. An adjustment means comprising a set screw 75 is used to press the frame 73 in the stepped portion 11 d against the projections 11 from the side of the container 11 through an arcuate press member 74. A recess 11f is provided in the side wall of the stepped portion 11 d to prevent the press member 74 from shifting in the circumference direction of the frame 73.
If the pressure applied by the set screw 75 to the frame 73 is deformed and this causes the curvature in the bottom surface 8a of the vibrating member toward the chamber 14 in a concave form. This causes the resilience of the resilient support mechanism to change. The compliance Cm of the diaphragm 10 consequently decreases, whereas the resonant frequency F according to Eq. (6) increases. Since the resonant frequency F may be changed by the set screw 75, the diaphragm 10 can be made to resonate at a given resonant frequency by changing the pressure applied by the set screw 75 even though the initial difference in performance resulting from variations in Cm, Mm impedes in the vibration of the diaphragm 10 in the transducer shown in Figures 15, 16 at a given frequency corresponding to a given fluid density. In other words, the difference in performance by types of transducer is easily nullified by adjusting the resilience of the resilient support mechanism 71 by the set screw 75, because the compliance Cm corresponds to the resilience of the resilient support mechanism 71. As a result, highly accurate measurement is possible without minimizing variations in the mass Mm and compliance Cm by specifically increasing its accuracy of the frequency measurement of the diaphragm 10.
In the above-described seventh embodiment, the diaphragm is supported by the resilient support mechanism and the means for sequentially changing the resilience of the resilient support mechanism, so that the difference in performance because of variations in the mass of the diaphragm and the spring constant in the diaphragm support is readily nullified by changing the resilience of the resilient support mechanism using the resilience-varying means. Accordingly, a highly accurate vibrating-type transducer is readily available without specifically increasing the accuracy of the frequency measurement of the diaphragm.
Figure 17 is a vertical sectional view of an eighth embodiment of the present invention, whereas Figure 18 is a drawing illustrative of the principal part of Figure 17. In both Figures 17, 18, the vibrating member 8 is equipped with piezo- electric vibrators 9 and 90 that are adhesion-bonded onto both inner and outer surfaces of the bottom 8a thereof, respectively. The piezo-electric vibrator 90 consists of a discoidal piezo-electric wafer 90A prepared from PZT material, a first electrode 90B and a second electrode 90C connected to both sides of the wafer 90A, respectively. The piezo-electric vibrator 9 consists of a discoidal piezo-electric wafer 9A prepared from the same material and having the same dimensions as that and those of the wafer 90A. The first electrode 9B is connected to one side of the wafer 9A, a second electrode 9C and a third electrode 9D connected to the other side thereof. Both the vibrators 9, 90 are fixed to the vibrating member 8 in such a manner as to conductively connect the first electrodes 9B, 90B and the vibrating member 8. In this case, the wafers 9A, 90A allow the electrodes 9B, 90B to be oppositely polarized as shown by an arrow P of Figure 18. Since member 8 and the vibrators 9, 90 are prepared from the above described materials the differences in the thermal expansion coefficient betweem them is extremely small.
The detection circuit will now be described. In Figure 18, an amplifier 91 applies output a.c. voltage to the piezo- electric wafers 9A, 90A through the electrodes 9C, 90C. A feedback circuit 92 detects the voltage generated in the piezo-electric wafer 9A through the electrode 9D and positively feeds it back to the amplifier 91. The diaphram 10 is thus constructed as shown in Figures 17, 18, and includes the piezo- electric wafers 9A, 90A which expand and contract in the radial direction when the a.c. voltage is applied across the electrodes 90B, 90C and 9B, 9C. When the piezo- electric wafers 9A, 90A thus expand and contract, the bottom 8a of the vibrating member 8 vibrates in the axial direction of the cylindrical body. In other words, the wafer 9A contracts in the radial direction when the wafer 90A expands in the radial direction, whereas the wafer 9A expands when the wafer 90A contracts, and the bottom 8a of the vibrator member 8 vibrates as described above. Because the polarities of the piezo- electric wafers 9A, 90A and those of the voltages simultaneously applied to the piezo- electric wafers 9A, 90A are arranged as described above, when the wafer 9A thus expands and contracts, a.c. voltage corresponding to the expansion and contraction of the wafer 9A is generated and supplied to the feedback circuit 91 through the electrode 9D. The diaphragm 10 resonates at the natural frequency F thereof and continues self-excited vibration because the output of the circuit is positively fed back to the amplifier 91. An impedance conversion circuit 93 receives output a.c. voltage having a frequency equal to the frequency F, whereas a waveform shaping circuit 94 shapes the waveform of the output signal of the conversion circuit 93 and supplies a pulse train signal of frequency F.
In the vibrating type transducer of Figure 3, the physical properties of each material vary although materials forming the vibrating member 8 and the vibrator 9 are selected as to minimize the difference between their thermal expansion coefficients. However, it is difficult to make the value of the difference therebetween small or keep it at zero over a wide range of operating temperatures. In consequence, the diaphragm 10 of Figure 3 may warp depending on the temperature at which the transducer of Figure 3 is operated and cause a measuring error to be produced, whreas the diaphragm 10 in the transducer shown in Figures 17, 18 will not warp over a wide range of operating temperatures because the vibrators 9, 90 equal in dimensions and materials are respectively bonded on both sides of the vibrating member 8. For this reason, such a transducer allows measurement with the least measuring error over a wide range of operating temperatures. In this case, the difference between thermal deformations resulting from the difference between the thermal expansion coefficients of the vibrating member 8 and the vibrators 9, 90 is absorbed by the adhesive sandwiched in between the vibrating member and the vibrators.
Figure 19 is a graph illustrating the result obtained from a test of the transducer shown in Figures 17, 18, wherein the line A represents a rate of change of the frequency of the pulse train signal appearing when the temperature of the fluid being monitored is changed. In Figure 19, the test result obtained from the embodiment of Figure 3 is also shown as the line B for the purpose of comparison. As shown in Figure 19, the temperature characteristics derived from the eighth embodiment of Figure 17 have been much improved as compared with that of the first embodiment of Figure 3.
The vibrator 9 or 90 formed on one side of the bottom 8a of the vibrating member 8 may be a pseudo-vibrator equal in dimensions and material to the vibrator 9 or 90 and bonded onto the other side thereof to constitute the diaphragm. This pseudo-vibrator is a disk without piezo-electric properties and electrodes. In the diaphragm thus formed, the effects of thermal expansion of the vibrator 9 or 90 and the pseudo-vibrator become equal. It is accordingly apparent that a transducer producing the least measuring error over a wide range of operating temperatures is obtainable as in the case of Figure 17.

Claims

1. A device for measuring the density or the pressure of a fluid on the basis of the resonant frequency of a vibrator comprising a container (11, 13) defining a cavity therein, a diaphragm (10) within said cavity dividing said cavity into two chambers (14, 15) on opposite sides of said diaphragm (10), said diaphragm (10) preventing flow communication between said chambers (14, 15), first fluid input means (11c) in flow communication with the first (14) of said chambers for introducing said fluid into said first chamber (14), second fluid input means (16) in flow communication with the other (secondi chamber (15) for introducing said fluid to said second chamber (15), said first and second chambers (14, 15) each having an acoustic compliance which is less than the mechanical compliance of said diaphragm (10), means (9, 21, 22) for causing said diaphragm (10) to resonate and means (23, 24) for detecting the resonant frequency of the diaphragm (10), characterized in that at least one (14) of said chambers (14, 15) together with its associated fluid input means (11c) is dimensioned to form a vibrating system (40) having a resonant frequency F which equals the natural frequency of the diaphragm (10).

2. The device of Claim 1, wherein said first fluid input means comprises a conduit having a cylindrical bore (11c) in flow communication with said associated chamber (14).

3. The device of Claim 2 further including means (62) for adjusting the length of said bore.

4. The device of any one of Claims 1 to 3, wherein said second fluid inlet means comprises second conduit having a cylindrical bore (30c) in flow communication with said second chamber (28).

5. The device of Claim 4 further including means (65) for adjusting the length of said bore (30c).

6. The device of Claim 1, wherein said first fluid inlet means (50) compruses a conduit having a convergent bore (51).

7. The device of Claim 6, wherein the shape of said bore (51) is exponential.

8. The device of Claim 6, wherein the shape of said bore is conical.

9. The device of any one of Claims 1 to 8, wherein said diaphragm (10) is rigidly affixed to said container (11, 13).

10. The device of any one of Claims 2, 6, 7 and 8 wherein said diaphragm (10) is affixed to a resilient member (73) and said device includes means (75) for adjusting the resilience of said resilient member (73).

11. The device of any one of Claims 1 to 10 wherein said diaphragm (10) is comprised of a piezo-electric vibrator (9) bonded to one side of a vibrating member (8) to induce vibration of said vibrating member (8).

12. The device of any one of Claims 1 to 10, wherein said diaphragm (10) is comprised of a piezo-electric wafer (9A, 90A) bonded on each opposite side of said vibrating member (8), at least one of said wafers (9A, 90A), being disposed to induce vibration of said vibrating member (8).

13. The device of Claim 12, wherein said piezo-electric wafers (9, 90) are affixed to said vibrating member (8) to minimize differential thermal expansion of said diaphragm (10).